I tweaked a custom color scheme to get my text area to look just the way I want it to---basically black text on a very light gray background. All is good there. But now when I open up any of the settings files (File Settings, Global Settings, Key Bindings, etc.), the words are light blue on that light gray background, and it's hard to read. (the separators are black, but the words are all light blue)

I can't for the life of me find out where the setting is that controls the color of that text in those settings files.

That should leave the other file types (like .sublime-settings) alone and only apply the desired color scheme to .txt files. Note that this is simply creating a new .sublime-settings file for the .txt file type. This new settings file will be named after the .tmLanguage file that defines the syntax def for .txt files. That's how syntax-specific settings work for every other file type.